1. Technical Field
This invention relates generally to an adaptive threshold circuit and, more particularly, to an adaptive threshold circuit for use with a magnetic or variable reluctance sensor that produces an alternating differential voltage in response to rotation of a wheel.
2. Discussion
Inductive magnetic sensors are commonly employed for automotive applications and the like to provide timing signals which enable the determination of position and speed of a rotating wheel. For example, specific applications may include the determination of engine crankshaft position and speed (i.e., RPM) or the determination of wheel speed for anti-lock braking systems. Inductive magnetic sensors used for these types of applications are commonly referred to as variable reluctance sensors.
The variable reluctance sensor is generally located adjacent to a rotating wheel which typically has a plurality of circumferentially spaced slots formed therein. The sensor has an inductive magnetic pick-up that is generally made up of a pick-up coil wound on a permanent magnetic core. As the wheel rotates relative to the pick-up coil, an alternating voltage is generated in the pick-up coil when the slots on the wheel move past the sensor. The frequency of the alternating voltage is then determined to achieve rotational speed information about the wheel.
The alternating voltage that is produced with the variable reluctance sensor has peak voltages that are proportional to the speed of rotation of the wheel. In a number of automotive applications, the peak voltage generated typically may vary from approximately 250 millivolts (mV) at low end speeds to over 160 volts (V) at higher speeds. In fact, some sensors are rated to generate up to 200 volts (V). In order to properly decode the sensor output, a receiving circuit is generally required to adapt the threshold voltage so that a processor or other control device may recognize a positive or high voltage level. Ultimately, the output signal may have to be in the form of a digital 0-5 volt pulse train, with which the processor may use the pulse width variation, for example, to determine the wheel speed or position.
In the past, a number of problems have existed which relate to noise that is created by external sources or defects in some component of the system. For instance, high voltage ignition coils may injects noise into sensor wires located nearby. Also, dirt or scratches existing on the surface of the wheel may cause noise which increases in proportion to the speed of the wheel. Other sources of noise may include manufacturing flaws in the wheel such as unequally formed slots in the wheel or off-center wheel alignment which may cause "wobble" or "run out". At high speeds, such noise could be several volts in amplitude and could cause false edges to occur at the output.
One proposed solution to the noise problem utilizes an analog circuit that has adaptive threshold control. One such analog circuit is discussed in U.S. Pat. No. 5,144,233 issued to Christensen et al. However, the analog adaptive control approaches generally have a number of drawbacks associated therewith. First, the analog approach commonly requires an accurate external capacitor to timely store a voltage as the next threshold. Second, unwanted noise spikes in the system can modify the charge stored on the capacitor which in turn can cause inaccuracies with the next input switchpoint. Third, processing variations may cause parametric changes in operation of the analog circuitry. Finally, conventional adaptive control approaches are designed for sensors which generate single-ended voltages, instead of a differential voltage.
One of the objects of this invention is to provide an adaptive threshold circuit that is capable of processing a voltage signal generated by a variable reluctance sensor over a selectable range.
Another object of this invention is to provide an adaptive threshold circuit that generates adaptive thresholds for determining the frequency of a detected signal while providing adaptive control signals for controlling an adaptive loading circuit with a differential voltage.
Still another more specific object of this invention is to provide for an adaptive threshold circuit which includes a counter and a digital-to-analog converter for generating a series of voltage pulses generated from a differential voltage produced by a variable reluctance sensor, and which does not require a capacitor of the type required for analog threshold circuits.